The present invention relates to a photometer circuit and more particularly to a reference voltage input control circuit for placing the photometer circuit in a reference voltage receiving and storing mode of operation and also more particularly to control, measurement and readout circuitry for controlling the operation of a photometer circuit in a comparing and measuring mode of operation by causing the photometer circuit to repeatedly measure, and then present a digital readout of the log of the reference voltage over the test voltage. The reference voltage is the voltage generated in a photosensor of the photometer circuit when light passing through a blank solution in a cuvette is received by the photosensor. The test voltage is generated in the photosensor by light passing through a liquid sample and received at the photosensor. The difference between the log of the reference voltage over the test voltage is indicative of the light absorbed by the liquid sample which in turn is indicative of the concentration of one or more chemicals in the liquid sample.
Heretofore, photometer circuits for measuring the log of a reference voltage over a test voltage have been proposed and such photometer circuits are disclosed in U.S. Pat. Nos. 3,566,133 and 3,752,995. The photometer circuit to be disclosed hereinafter includes several of the basic circuit components of the photometer circuit disclosed in U.S. Pat. No. 3,752,995 and includes additional circuits which constitute improvements to the photometer circuit disclosed in U.S. Pat. No. 3,752,995.